The present invention relates to a process of producing a polymer optical waveguide that can be suitably used in optical circuits, optical switches, optical multiplexers/demultiplexers, etc.
With the progress of practical application of optical communication system by development of optical fibers, development of various optical communication devices using an optical waveguide structure is demanded. In general, optical waveguide materials are required to have characteristics such that they have a low light propagation loss, have heat resistance and humidity resistance, and can control a refractive index and a film thickness. In recent years, use of synthetic resins as the optical waveguide materials is investigated. Of a number of synthetic resins, polyimide resins have the highest heat resistance and therefore, recently attract a great deal of attention.
Hitherto, optical circuits comprising a polyimide resin have been generally formed by the following dry process. That is, a polyamic acid as a polyimide resin precursor is first dissolved in a polar solvent such as N,N-dimethylacetamide and N-methyl-2-pyrrolidone, to prepare a polyamic acid varnish; the varnish is coated on a substrate by spin coating or casting; the coated varnish is heated to not only remove the solvent but also cyclize the polyamic acid for imidation, thereby forming a polyimide resin film; and the polyimide resin film is then subjected to reactive ion etching (RIE) with oxygen plasma, etc., to form a pattern as an optical circuit on the polyimide resin film.
However, according to such a conventional dry process, not only it takes a long period of time to form an optical circuit, but also the problem of realizing a reduction of the production cost is not yet solved because the processing region is restricted. Further, according to such a dry process, since a wall surface (side surface) of the formed pattern is not flat, there is a problem such that a scattering loss becomes large during wave guiding of a light into the optical circuit.
On the other hand, a method of forming a pattern by a wet process of a photosensitive polyimide resin precursor composition, i.e., a photosensitive polyamic acid resin composition, namely a method in which a region of a photosensitive polyamic acid resin composition layer corresponding to a required pattern is irradiated with UV light and then developed to obtain a pattern corresponding to the UV light-irradiated areas, is also known (see, for example, JP-A-2-201322).
For preparing an optical waveguide utilizing this method, for example, an undercladding layer 2 is formed on a substrate as shown in FIG. 1A; a photosensitive polyamic acid resin composition layer 3 is formed on the undercladding layer 2 as shown in FIG. 1B; and the photosensitive polyamic acid resin composition layer 3 is then irradiated with UV light through a glass mask 4 so as to obtain a required pattern as shown in FIG. 1C. The photosensitive polyamic acid resin composition layer is then treated with a developing solution to leave UV light-exposed areas and remove UV light-unexposed areas, thereby forming a core layer 5 having the required pattern on the undercladding layer 2 as shown in FIG. 1D. Subsequently, an overcladding layer 6 is formed on such a core pattern to obtain a channel type optical waveguide as shown in FIG. 1E.
According to the wet process including UV light irradiation of a photosensitive polyamic acid resin composition and a subsequent development step, although the wet process is free from the above-described problems in the dry process, the wet process involves a new problem with respect to a pattern obtained. That is, according to the wet process including a development step, since circulation of a developing solution varies depending on the shape of an etching tank and the pattern of an actual sample so that the patterning property is greatly influenced, it is difficult to obtain a desired pattern with high precision and good reproductivity. Further, there may be the case where impurities in the developing solution adhere to the desired pattern, leading to partial etching failure.
In addition, according to the foregoing wet processing including a development step, since the UV light-unexposed areas are removed to form a core layer corresponding to the UV light-exposed areas, when the overcladding layer is formed on the core pattern, another problem involves such that a difference in level based on the core layer must be made uniform to flatten the surface of the overcladding layer. Especially, in the case where an overcladding layer comprising a polyimide is formed on the core layer, since the polyimide is in general formed by coating a solution of a polyamic acid as its precursor on a pattern, heating and then curing (imidating), shrinkage in volume by volatilization of a solvent as used is large so that it is difficult to make a difference in level ranging from about 5 to 10 xcexcm as formed by the core layer uniform to flatten the surface of the overcladding surface.
Also, a process of producing a polymer optical waveguide by a wet process not including a development step is already known. That is, this process comprises applying a heat/UV-light curable resin with a high refractive index containing a photopolymerization initiator and a heat polymerization initiator on a substrate to form a resin layer; irradiating the resin layer with light to undergo crosslinking to form a core portion; heating the resin layer to undergo heat curing of UV light-unexposed areas to form a cladding portion comprising a thermosetting resin with a lower refractive index on the side surfaces of the core portion; and further forming an upper cladding portion on the resin layer comprising a core portion and a cladding portion, to produce an optical waveguide (see JP-A-2002-71987).
According to such a process, nevertheless the wet process, it does not include a development step. Accordingly, the process is free from the above-described problems. On the other hand, since a resin to be used must have photopolymerization property and heat polymerization property, resins that are to be used are very restricted.
As a result of extensive and intensive investigations to overcome the above-described problems in the conventional production of polymer optical waveguides by a wet process, it has been found that when a 1,4-dihydropyridine derivative as a photosensitive compound is compounded with an arbitrary resin to form a photosensitive resin composition, which is then irradiated with light and heated, a residual component of a decomposition product of the 1,4-dihydropyridne derivative and its amount influence a refractive index of a resin layer, resulting in enabling to make an effective difference in refractive index between exposed areas and unexposed areas of the resin composition layer and that nevertheless a wet process not including a development step, a polymer optical waveguide structure can be thus formed using an arbitrary resin. The present invention has been completed based on this finding.
Accordingly, an object of the invention is to provide a process of producing a polymer optical waveguide, in which nevertheless a wet process not including a development step, an overcladding layer can be formed in a flat shape on a core layer, and a resin to be used can be freely chosen.
Specifically, the invention provides a process of producing a polymer optical waveguide, which comprises:
(a) a step of forming an undercladding layer on a substrate;
(b) a step of forming a photosensitive resin composition layer containing a 1,4-dihydropyridine derivative and a resin on the undercladding layer;
(c) a step of irradiating a region of the photosensitive resin composition layer corresponding to a core pattern with UV light through a mask to form UV light-exposed areas and UV light-unexposed areas on the photosensitive resin composition layer;
(d) a step of heating the UV light-exposed areas and UV light-unexposed areas of the photosensitive resin composition layer; and
(e) a step of forming an overcladding layer on the photosensitive resin composition layer after heating.